MIMO wireless communication systems comprise multiple-antenna wireless communication devices on both ends of a communication link, e.g., at an access point (AP) and a client station (CS). MIMO wireless communication techniques can enhance the radio link reliability and increase the system capacity through diversity gain and multiple signal stream transmission.
Beamforming MIMO communication techniques can further improve the downlink throughput and reliability from an AP to a CS, but beamforming requires downlink channel state information (CSI). Obtaining CSI through channel reciprocity is useful in some situations because it does not require the overhead of a specific feedback signal from a CS. However, in order to obtain CSI through channel reciprocity, some MIMO wireless communication systems require the CS to send one or more sounding frames periodically on the uplink to the AP. Sounding frames introduce additional preamble overhead and not all CSs, especially lower cost CSs, are capable of transmitting such sounding frames. As a result, the AP cannot perform implicit beamforming of multiple signal streams to those CSs even though the AP is otherwise beamforming MIMO capable.
It would be desirable to configure an AP to derive the entire multidimensional channel information between it and a CS that is not capable of sending sounding frames. This will allow the AP to beamforming multiple streams to such lesser-capable CSs.